Sensor head for ACFM based crack detection

Abstract

A sensor head with multiple sensors for ACFM based crack detection and sizing. An alternating current electromagnetic yoke is used to induce a uniform magnetic field on the surface of the test piece. A reference coil in the sensor head detects the background field strength of the magnetic field. Multiple sensors, the axis of each of which is in an angled relationship to the reference coil, detect variations in the magnetic field. The sensor coil axes can be chosen to detect and size both angled and straight cracks.

What is claimed is:  
     
       1. A sensor head for detecting variations in a magnetic field induced on the surface of a test piece by an electromagnetic elongated yoke, the sensor head comprising 
       a reference coil for sensing magnetic fields produced by the current, the reference coil having a longitudinal axis parallel to a longitudinal axis of the elongated yoke, and the reference coil being disposed between the yoke and the test piece;  
       a plurality of sensor coils for sensing variations in magnetic fields, each sensor coil having a longitudinal axis which intersects the longitudinal axis of the reference coil;  
       each sensor coil being positioned between the reference coil and the yoke; and  
       mounting means for mounting the reference coil and the plurality of sensor coils in the sensor head  
       wherein at least one of the plurality of sensor coils has a longitudinal axis not perpendicular to the longitudinal axis of the reference coil. 
     
     
       2. A sensor head as claimed in  claim 1  wherein the longitudinal axis of at least one sensor coil is perpendicular to the longitudinal axis of the reference coil. 
     
     
       3. A sensor head as claimed in  claim 1  wherein the longitudinal axis of at least one sensor coil is perpendicular to the longitudinal axis of the reference coil, and perpendicular to the surface of the test piece. 
     
     
       4. A sensor head is claimed in  claim 1  wherein the plurality of sensor coils comprises three sensor coils. 
     
     
       5. A sensor head as claimed in  claim 1  wherein the plurality of sensor coils comprises five sensor coils. 
     
     
       6. A sensor head as claimed in  claim 1  wherein an equal number of sensor coils is positioned on each side of a plane containing the longitudinal axes of both the reference coil and the yoke. 
     
     
       7. A sensor head as in  claim 1  wherein at least one sensor coil is movable and lockable in place. 
     
     
       8. An ACFM testing module for testing a test piece, the module comprising: 
       an electromagnetic elongated yoke;  
       a reference coil having a longitudinal axis parallel to a longitudinal axis of the yoke, the reference coil being disposed between the yoke and the test piece;  
       three sensor coils, each sensor coil having a longitudinal axis which intersects the longitudinal axis of the reference coil and each sensor coil being positioned between the reference coil and the yoke;  
       power supply means providing power to the yoke;  
       measurement means coupled separately to the reference coil and to each sensor coil; and  
       mounting means for mounting the reference coil and the three sensor coils in a sensor head.  
     
     
       9. An ACFM testing module as claimed in  claim 8  wherein at least one of the three sensor coils is movable and lockable in place. 
     
     
       10. A sensor head for use with ACFM crack detection and sizing and for use with an electromagnetic elongated yoke inducing a current sheet in a test piece, the head comprising: 
       a reference coil having a longitudinal axis parallel to a longitudinal axis of the yoke, the reference coil being positioned between the test piece and the yoke;  
       three sensor coils, each sensor coil having a longitudinal axis at an angle to the longitudinal axis of the reference coil, each sensor coil being positioned between the reference coil and the yoke; and  
       mounting means for mounting the reference coil and the three sensor coils in the sensor head;  
       wherein the longitudinal axes of the three sensor coils are located in a plane perpendicular to the longitudinal axis of the reference coil. 
     
     
       11. A sensor head as in  claim 10  wherein the longitudinal axis of a first sensor coil of the three sensor coils is perpendicular to the longitudinal axis of the reference coil, and perpendicular to a surface of the test piece. 
     
     
       12. A sensor head as in  claim 10  wherein at least one of the at least three sensor coils is movable and lockable in place. 
     
     
       13. A method of detecting and sizing defects in or near a surface of a test piece, said method comprising: 
       a) energizing a yoke to induce a uniform alternating current on the surface of the test piece  
       b) positioning a reference coil such that a longitudinal axis of the reference coil is parallel to a longitudinal axis of the yoke  
       c) positioning at least two sensor coils with their axes at an angle to the reference coil axis at least one sensor coil axis not being perpendicular to the reference coil axis  
       d) moving the sensor head across the surface of the test piece  
       e) detecting variations in electromotive force induced in the reference coil  
       f) measuring the variations detected in step e)  
       g) detecting variations in electromotive force induced in the sensor coils  
       h) measuring the variations detected in step g)  
       i) processing data obtained in steps f) and h) to determine a presence and a size of a defect on the surface of the test piece.  
     
     
       14. A method as claimed in  claim 13  wherein the sensor head comprises: 
       a reference coil for sensing magnetic fields produced by the current, the reference coil having a longitudinal axis parallel to a longitudinal axis of the yoke, and the reference coil being disposed between the yoke and the test piece,  
       a plurality of sensor coils for sensing variations in magnetic fields, each sensor coil having a longitudinal axis which intersects the longitudinal axis of the reference coil and each sensor coil being positioned between the reference coil and the yoke, and  
       mounting means for mounting the reference coil and the plurality of sensor coils in the sensor head.